Topband: chicken wire ground screen (reply)

[Tom Rauch]

> Gary, K9AY is quite correct in his response that [copper] mesh used as
> an artificial ground at HF and VHF has proven quite effective and that
> radar and satellite antennas use perforated reflectors to save weight
> [I presume Gary is refering to aluminum mesh reflectors in this case
> due to weight concerns].
> 
> However, I interpreted the original point of discussion to be related
> to ferromagnetic materials, such as typical chicken wire, fence wire,
> etc., which is usually uncoated or galvanized soft-iron wire. 

Disconnecting or connecting a wire makes no difference at all, as 
long as you have sufficient wire in the ground. If you have an 
inadequate radial ground, the interaction is quite complex and very 
unpredictable. Best to just bite the bullet and install a decent 
radial ground, or the biggest ground system you can, and tie it all 
together. 

The only effect of large areas (large compared to skin depth) steel 
or iron in RF fields is to increase resistive losses. The magnetic 
field from the antenna does not see the "magnetic" properties of the 
iron, except as the iron content slightly reduces conductivity of the 
wire and influences skin depth.

All in all, it doesn't matter much what the material is except for 
life of the counterpoise as long as you have sufficient counterpoise 
area.

One easy demonstration of the lack of magnetic concentration in the 
iron is to insert a sample inside an air-core inductor and measure 
the inductance at 1.8MHz, or whatever frequency is in question. What 
you will find is the inductance is **reduced** by inserting a solid 
iron or steel wire inside an HF inductor, just as it is with non-
magnetic materials like brass or aluminum.

Skin effect and the resistivity, by far, is the only electrical 
concern.

The real important point (other than life or other physical problems) 
is the screen must have sufficient distance out from the base of the 
antenna to reach the same radius of all the missing radials, and must 
have good physical coverage of the lossy media below the screen 
(soil).

A big very wide conductor or screen near the antenna, or even several 
of them sticking out 1/10th of a wave, does NOT make a decent ground 
system. 5,280 feet of wire in a shoe box is still electrically a shoe 
box-sized one-foot ground, not a mile of ground.

That is the important point.



               73, Tom W8JI
W8JI@contesting.com